1. Field of the Invention
The present invention concerns an improved method of operating burners that use flue gas recirculation (FGR). The improved method measures changes in air and air-flue gas mixture temperatures to estimate density variations, and then uses this information to optimize the performance of a combustion air fan over a range of air and recirculated flue gas mixture ratios and temperature ranges. The method can be applied to any burner that uses FGR, as long as the amount of flue gas that is used can be varied at some or all operating conditions without extinguishing the flame. Advantages of this technique are smaller fan size with associated lower first cost for a given maximum burner thermal capacity, reduced electrical power usage over the full range of burner operation, and a lower peak power requirement at a maximum burner capacity.
2. Description of Related Art
Over the past 30 years, the cost of owning and operating combustion equipment, e.g. boilers, has increased due to more stringent environmental regulations and, more recently, due to higher electricity prices. One impact of environmental regulations has been the requirement to use ultra-low NOx burners (hereinafter ULNBs) to achieve sub-9 ppm NOx emissions requirements. While the best ULNBs can match the thermal efficiency of standard burners, electric power usage is typically higher. Conventional ULNBs now often require over twice the fan power that was required of previous generation 30-ppm low NOx burners (hereafter LNBs).
Most fans used in industrial boilers and process heaters are constant speed fans, and a constant speed fan moves air at a fixed maximum volumetric flow rate. As air density changes, the mass flow of air changes while the volumetric flow stays the same.
Power consumption of a combustion air fan is proportional to the pressure rise created by the blower multiplied by the volumetric flow rate. With pressure rise measured in units of lbf/ft2 and flow measured in units of ft3/sec it is apparent that the product of these two numbers has units of lbf-ft/sec, which are units of power.
xe2x80x83Powerxe2x88x9dxcex94Pxc2x7Qxe2x80x83xe2x80x83(1)
In a fixed boiler and burner geometry, the pressure requirements of the combustion air fan will vary in proportion to the product of density and the square of the velocity. Therefore, if density remains constant the pressure drop will increase with the square of an increase in mass flow.
xcex94Pxe2x88x9dxcfx81u2 xe2x80x83xe2x80x83(2)
If mass flow remains constant and density changes, then {dot over (m)}xe2x88x9dxcfx81u remains constant, and pressure will change linearly with the inverse of density.       Δ    ⁢          xe2x80x83        ⁢    P    ∝                              m          ·                2            ρ        ⁢          (              fixed        ⁢                  xe2x80x83                ⁢        geometry            )      
Volumetric flow will change linearly with mass flow if density is constant, and linearly with specific volume (inverse of density) if mass flow is constant.   Q  =            m      ·        ρ  
Increasing mass flow at constant air density therefore increases fan power requirements in proportion to the mass flow ratio raised to the third power.       Power    ∝                            m          3                          ρ          2                    ⁢              (                  fixed          ⁢                      xe2x80x83                    ⁢          geometry                )            ⁢              xe2x80x83            ⁢      by      ⁢              xe2x80x83            ⁢      equations      ⁢              xe2x80x83            ⁢              (        1        )              ,      (    3    )    ,      and    ⁢          xe2x80x83        ⁢          (      4      )      
For a fixed mass flow, decreasing air density by increasing the temperature (for example by replacing cool ambient air with the equivalent mass of warm flue gas) will increase fan power requirements in proportion the square of the specific volume change, equation (5). Lower density air uses more fan power for a fixed mass flow.
While the fan behaves as a constant volume device, the burner is not. At a fixed heat input, a burner is more accurately described as a constant mass flow device. In order to maintain a fixed fuel-air ratio (or fixed xe2x80x9cdilution levelxe2x80x9d) at a fixed fuel flow, a constant mass flow of air is required, not a constant volume flow. Many ULNB""s require that a nearly constant xe2x80x9cdilution levelxe2x80x9d be maintained in order to maintain a given NOx emissions level.
Combustion systems use excess air to ensure complete combustion of the fuel, and importantly in some burners, to lower the combustion temperature to minimize NOx formation. Excess air is conventionally defined as the amount of air that is in excess of the stoichiometric requirement of the fuel with which it is mixed. Good practice calls for an excess air level of 15% or greater. For burners operating at 9 ppm (parts per million on a volumetric basis) or lower NOx emissions, the excess air level may be 65% or higher. Most of the excess air serves to lower the combustion temperature and hence its oxygen content acts as an inert like nitrogen to lower combustion temperature.
Flue gas is warmer than air, and it is thermally more efficient to recirculate some flue gas in place of some of the excess air in high-excess-air burners. This can be done as long as the oxygen-depleted flue gas is not mixed with air in a proportion that makes the mixture have insufficient oxygen for complete combustion of the fuel. If flue gas is substituted for an equivalent amount of air on a mass basis, it has been shown that similar burner emissions with be achieved. This is because the primary effect of the addition of either flue gas or excess air to the burner is to quench the flame and to reduce NOx formation. Therefore, controlling the xe2x80x9ctotal dilutionxe2x80x9d of the burner, where xe2x80x9ctotal dilutionxe2x80x9d is defined as the total mass flow of air and flue gas in excess of stoichiometric air, can be an effective way of controlling emissions.
From the above summary, it is seen that increasing mass flow and increasing the temperature of the mass flowing through a burner will increase the power required to achieve a specific firing rate at a fixed fuel-air or dilution level. Large increases in mass flow associated with ULNBs utilizing FGR have had a significant detrimental impact on fan power requirements as will be show below.
Power requirements of burners with different NOx emissions, different FGR levels, and different dilution levels are discussed in the following paragraphs and summarized in Table 1. To put the issue of fan power into perspective, one starts with an uncontrolled gas-fired burner with about 100 ppm NOx emissions, before evaluating a typical 30 ppm burner and then lastly two different 9 ppm ULNBs.
A standard natural gas fired burner with uncontrolled NOx emissions is designed for optimum performance at about 15% excess combustion air. When installed in a standard industrial boiler, a 100 MMBtu/hr burner operating at 15% excess air with 100 ppm NOx emissions would require approximately a 50 hp fan, or 0.5 fan hp per MMBtu/hr of boiler capacity.
In the late 1980""s and early 1990""s, 30 ppm NOx regulations were introduced, and the method of choice for achieving this emissions level was to add typically 15% FGR to a burner operating at 15% excess combustion air. Burners that operate at these, or similar, FGR and excess air levels have now been in widespread use for over 20 years. While this approach has proven to be a good method of achieving 30 ppm emissions, larger combustion air fans and fan motors were required. The combined mass flow of air and flue gas was increased by 15% (with the addition of 15% FGR to the air). In addition the volumetric flow of gas through the fan and burner was increased due to the decrease in density. With 15% FGR, the temperature increase was on the order of 40xc2x0 F., which corresponds to a density decrease of 8%. Fan power requirements increased by 1.153/0.922 (equation 5) or a factor of 1.80. Where a 50 hp fan worked for a 100 ppm burner, now the end user needed about 90 hp (0.90 fan hp per MMBtu/hr of capacity).
More recently, 9 ppm and lower emissions regulations have been introduced for natural gas fired burners in boilers and process heaters. Alzeta Corp. of Santa Clara, Calif. sells a burner for use in food processing and other industries that utilizes only excess combustion air (no FGR) to achieve the flame dilution necessary for 9-ppm NOx emissions. A dilution level of 60% on a mass basis is required, but since no FGR is used, there is no change in the density of the flow through the fan and burner. This 60% dilution represents a 40% increase in mass flow relative to the baseline burner with 15% dilution. Fan power requirements are therefore increased by 1.43 (equation 5) compared to the uncontrolled baseline, or a factor of 2.74. To achieve 9 ppm NOx in a 100 MMBtu/hr boiler, the end user would require about 137 hp (1.37 hp per MMBtu/hr of capacity). One disadvantage of the high excess air approach is that thermal efficiency of the boiler is reduced relative to a boiler that uses FGR.
When FGR is used to reach the 9 ppm emissions level, the typical ULNB uses about 35% FGR, 20% excess air, and a total increase in mass flow of about 40% (Equivalent mass flow to what is required by the high excess air burner). FGR increases the volumetric flow of gas through the fan and burner due to the high xe2x80x9cmix temperaturexe2x80x9d of the 35% FGR in ambient air. Assuming a stack temperature of 500xc2x0 F., the mixed temperature of a 35% FGR flow in 60xc2x0 F. air is 175xc2x0 F., which causes an 18% decrease in density relative to air at standard conditions. Fan power requirements increase by 1.43/0.822 (eqn 5) or a factor of 4.08. Where a 50 hp fan was sufficient for the uncontrolled baseline, now a 204 hp fan is required. (2.04 fan hp per MMBtu/hr capacity). This huge increase in fan power requirements has been documented at many facilities throughout the nation.
This growth in fan power requirements to meet lower emissions levels has had a number of adverse effects on end users. These adverse effects include increased capital costs and higher operating costs.
One method of reducing fan power requirements is to use a variable frequency drive (VFD) instead of a constant speed fan. VFD systems have higher capital cost, require more sophisticated burner controls, and are therefore used much less often than constant speed fans in industrial boilers and heaters. End users choosing to use a VFD could benefit from this invention, but to a lesser degree than users of constant speed fans. This invention when used with a VFD would allow the use of a smaller fan housing and would provide increased thermal turndown.
Another method of reducing fan power is to use a 30-ppm LNB and then use stack treatment technology to reduce NOx emissions to the 9-ppm level. This approach has been shown to have much higher capital cost than the ULNB systems in almost all cases. Therefore, the use of stack treatment technologies with industrial boilers is extremely rare. As with the VFD example described above, the stack treatment system, if preferred, could benefit from the use of the present invention to improve performance of a 30-ppm burner as long as the 30-ppm burner used FGR.
Currently, many well known manufacturers of ULNBs are operating at similar levels of total dilution. These commercial manufacturers include Alzeta, of Santa Clara, Calif.; Todd Combustion (a John Zink Company) of Tulsa, Okla.; and COEN of Burlingame, Calif. Since the fundamental principal involved in reducing NOx is to quench the flame, it is not surprising that similar total dilution levels are required. The nominal air and flue gas flows that are required are 20% excess combustion air and 35% FGR to achieve 9 ppm emissions. This total flow is typically moved through a single large combustion air fan. Currently only the inventors have recognized the improvement as described below for the present invention.
Art of interest in this technology includes but is not limited to the following:
P. K. Nelson et al., U.S. Pat. No. 4,659,305, assigned to AquaChem., Inc., describes a flue gas recirculation system for fire tube boilers and burners.
R. E. Lofton et al., U.S. Pat. No. 5,040,470, assigned to Shell Western E and P, Inc., describes a steam generation system with NOx reduction.
E. L. Berger et al., U.S. Pat. No. 6,247,917, assigned to Texaco, Inc., discloses a flue gas recirculation system.
J. D. Sullivan, et al. in U.S. patent application Ser. No. 10/423,680 filed Apr. 25, 2003 discloses a temperature compensated combustion control.
J. D. Sullivan, et al. in U.S. patent application Ser. No. 10/424,047 filed Apr. 25, 2003 discloses a method of combustion control with temperature compensation.
However, none of these cited references individually or in combination in any fashion teach or suggest the present invention.
All patents, patent applications, articles, references, standards, components, supplies and the like cited herein are incorporated herein by reference in their entirety.
From the above description, it is apparent that a need exists for an apparatus and method for cost effectively reducing the fan size and power requirements of burners that use FGR.
The present invention may use previous inventions of the inventors that accurately control the mass flow of a diluent stream over a broad range of diluent temperature. Having the ability to control diluent flow on a mass flow basis allows for the proper operation of this invention. This invention improves the operation of the burner by making more effective use of the combustion air fan over the full range of burner operation. More specifically, the invention is a burner that operates with a lower temperature oxidizer stream at high mass flow conditions to provide a system that has less overall variation in volumetric flow and fan power requirements over the full burner operating range.
This improvement is achieved by using this invention to improve the utilization of flow and power characteristics of a combustion air fan over the full range of burner operation. More specifically, a method of operation is described that uses a high recirculated flue gas-to-air ratio over most of the boiler operating range to maximize thermal efficiency, but which switches to a lower ratio of flue-gas-to-air, or no flue gas in the limiting case, at the high end of the mass flow range of the burner to minimize the size of the fan housing and fan motor and power requirements. When applied to current generation ULNBs, fan size and power consumption of the fan can be reduced by about 25% without adversely affecting overall NOx emissions.
If the percentage of flue gas relative to fresh combustion air is increased from 35% to 50% to achieve lower NOx emissions or higher thermal efficiency, then the reduction in power due to the benefits of this invention could be 35% or higher. Similarly, if the temperature of the flue gas was higher than 500xc2x0 F. assumed as a typical condition in this analysis, then the power savings would also be greater than 25% and would increase with increasing stack temperature.
In one aspect, the present invention concerns an improved method which increases the operating range of a combustion burner that controls the ratio of recirculated flue gas to air in a flue gas-air oxidizer stream which method comprises:
contacting the combustion generator with fuel and full excess air, recirculated flue gas, and combinations thereof wherein the modulation and control of the flue gas-air ratio is useful to reduce the size of at least one fan in the recirculated flue gas line, one fan in the air line or combinations thereof and results in reducing the power requirements of at least one fan and reduces the emission of nitrogen oxides.
In another embodiment, the present invention is an improved method for operating gas, liquid or solid fuel combustion burners or combinations thereof that utilizes recirculated flue gas, (full) excess air and combinations thereof which method increases the operating range of the burner by operating from a high recirculated flue gas percentage at lower heat input to a lower recirculated flue gas percentage at high heat input to minimize the change in oxidizer volumetric flow relative to change in the oxidizer mass flow, which method comprises (with reference numbers keyed to FIGS. 1, 2, 2A, 2B and 2C):
a. contacting a combustion generator (or burner) (3) with a combustible fuel (4A), wherein said combustion generator is located within a combustion chamber (3A);
b. utilizing an air inlet (10) to provide excess air (10A) to the combustion generator (3);
c. utilizing an exhaust stack (3Q) for exhausting flue gas from the combustion chamber (3A) wherein said exhaust stack has a stack inlet coupled to the combustion chamber (3A) and a take-off point;
d. utilizing a recirculation inlet (3T) and a recirculation outlet (11) for flue gas (11A) wherein said recirculation inlet (3T) is coupled to said take-off point;
e. combining in a mixing zone (10B), recirculated flue gas (11A), air (10A), and combinations thereof for transport of recirculated flue gas and air through the burner; and
f. contacting the combustion generator with fuel and recirculated flue gas-air mixtures including up to 100% air, wherein the modulation control between recirculated flue gas-air and excess air mixtures have flow control means (7) for controlling the oxidizer flow between the recirculated flue gas and excess air ratio during operation to result in a mass of diluent which is provided to the said combustion chamber that results in the reduced fan size about 50% or less of conventional and the operational electrical power needed for the at least one fan is about 50% or less of a comparable fan for a conventional combustion system.
In another embodiment in step e the mixing zone is selected from:
(i) a common fan (71) having a recirculated flue gas inlet (72), an air inlet (73) and an outlet (74) to the combustion chamber (3);
(ii) a chamber (81) having a flue gas recirculation line (82) which contains at least one fan (82A) and a separate air line (83) which contains at least one fan (83A) and an outlet line (84); or
(iii) an air line (93) which includes at least one fan (93A) connected to the combustion chamber (3) and a flue gas recirculation line (92) which includes an optional fan (92A) having a fuel line (95) immediately downstream of the optional fan (92A) which line (96) exits to the combustion chamber (3).
In a preferred embodiment, in step f the control of the recirculated flue gas-air ratio utilizes an adjustable damper or an on-off damper in the flue gas recirculation line.
In a preferred embodiment, in step e the at least one fan is selected from the group consisting constant speed, variable frequency drive and combinations thereof.
In a preferred embodiment, in step e the means for flue gas-air ratio control is selected from manual, electrical motor, or electro-pneumatic modulation.
In a preferred embodiment, in step e the means for modulation control is analog or digital control modulation.
In one preferred embodiment, in step a the combustion generator uses gas, liquid or solid fuel.
In a preferred embodiment, in step a the combustion fuel is natural gas and in step e the means for modulation of recirculated flue gas-air ratio is digital control and the level of nitrogen oxides emitted is about 9 ppm or less.
In a preferred embodiment, in step e the means for control is selected from an adjustable damper, an on-off damper, a transfer fan, or combinations thereof, and the level of nitrogen oxides emitted is less than about 9 ppm.
In a preferred embodiment, in step a the fuel is natural gas, in step c a fan in each line is present as described in subpart (ii), and the air-recirculated flue gas ratio modulation utilizes analog or digital control.
In another aspect, the present invention concerns an apparatus for a gas, liquid, solid fuel combustion burner or combinations thereof that controls the ratio of flue gas to air in a flue gas-air oxidizer stream during operation which is useful to reduce the size of at least one fan in the flue gas recirculation line, air line or combinations thereof and reduce the power consumption of at least one fan in the flue gas recirculation line, air line or combinations thereof which concurrently increases the operating range of the apparatus.
In a preferred embodiment, the present invention concerns an apparatus for operating gas, liquid, solid fuel combustion burners or combinations thereof that utilizes recirculated flue gas, excess air and combinations thereof which apparatus increases the operating range of the burner by operating from a high recirculated flue gas percentage at lower heat input to a lower recirculated flue gas percentage at high heat input to minimize the change in oxidizer volumetric flow relative to change in the oxidizer mass flow, which apparatus comprises:
A. a combustion generator (3) located within a combustion chamber (3A or 3R),
B. an exhaust stack (3Q) for exhausting flue gas (11A) from the combustion chamber (3) wherein said exhaust stack (3Q) has a stack inlet coupled to said combustion generator and a take-off point (3T),
C. a recirculation inlet (3T) and a recirculation outlet (11B) wherein for flue gas said recirculation inlet is coupled to said take-off point (3T),
D. a separate air inlet (10) to provide excess air (10A) to said combustion generator (3) for transport of recirculated flue gas and air through duct (10D) the burner (3);
E. a mixing zone (10B or 10D or 71A or 81) for combining flue gas (11A), air (10A), and combinations thereof;
F. means within the flue gas recirculation line for modulation control (12 and 13 or 12 and 11) between recirculated flue gas (11A) and excess air (10A) to result in reduced fan size of about 50% or less and in operational electrical power of about 50% or less than the conventional for the at least one fan needed for full excess air.
In another embodiment, the present invention includes in component E a mixing zone selected from:
(i) the mixing zone (71A) is a fan chamber (71) including a mixing fan said chamber has a recirculated flue gas inlet (72), an air inlet (73) and an outlet (74) to the combustion chamber (3),
(ii) the mixing zone (81) has a recirculated flue gas inlet line (82) which contains at least one fan (82A), an air line (83) which contains at least one fan (83A) and an outlet line (84) to the combustion chamber (3), or
(iii) the mixing zone includes an air line (93) which includes at least one fan (93A) and connects to the combustion chamber (3), and a flue gas recirculation line (92) which includes an optional fan (92A) having the fuel line (95) immediately downstream of the fan (92A), having an exit line (96) to the combustion chamber (3).
In another embodiment, the present invention concerns an improved method for operating gas, liquid, or solid fuel burners that utilizes recirculated flue gas-air and excess air mixtures which method increases the operating range of the burner by operating from a high recirculated flue gas percentage at lower heat input to a lower recirculated flue gas percentage at high heat input under conditions to minimize the change in oxidizer volumetric flow relative to change in the oxidizer mass flow, which method comprises:
a. contacting a combustion generator with a combustible fuel, wherein said combustion generator is located within a combustion chamber;
b. utilizing an air inlet to provide up to full excess air to said combustion generator;
c. utilizing an exhaust stack for exhausting flue gas from the combustion chamber wherein said exhaust stack has a stack inlet coupled to the combustion generator and a take-off point and;
d. utilizing a recirculation inlet and a recirculation outlet for recirculated flue gas wherein for said recirculation inlet is coupled to said take-off point; and
e. contacting the combustion generator with excess air and recirculated flue gas-air mixtures, wherein said switching between full excess air and recirculated flue gas-air mixtures to result in a mass of air which is provided to the said combustion chamber and a results in a reduced fan size of about 50% from conventional and reduced electrical power needed for fans to provide full excess air or recirculated flue gas is about 50% or less of comparable conventional combustion systems.
An embodiment of this method wherein the means for control of the flue gas-air ratio is selected from an adjustable damper, an on-off damper, digital control, analog control or combinations thereof.
The other embodiments listed above for the apparatus are incorporated by reference here for the method.